1. Field of the Invention
This invention relates to a process for making polymers, and, more particularly, to a polymerization process using t-amylperoxy pivalate as the free radical initiator.
2. Description of the Prior Art
Free radical initiators used in polymerization processes at elevated temperatures have included alkyl peresters, dialkyl peroxides, perketals, peroxydicarbonates, hydroperoxides, azo compounds and carbon-carbon labile compounds. Copolymerization also has been carried out at relatively low temperatures, e.g. room temperature, using a redox catalyst, such as, a combination of a redox catalyst and ascorbic acid, or, of a peroxydicarbonate and benzoin. It has been possible also to employ, the polymerization initiator conjointly with a suitable accelerator, for example, an amine derived from a cyclic or phenyl structure, the amine being used by itself or together with an organic compound of a transition metal. Copolymerization also has been carried out with a mixture of different polymerization initiators having different decomposition temperatures.
The following compounds have been employed in free radical initiated polymerizations: acetyl cyclohexanesulfonyl peroxide, diacetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, tert-butyl per[oxy]-neodecanoate, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile); tert-butylperoxy pivalate, dioctanoyl peroxide, dilauroyl peroxide, 2,2'-azobis-(2,4-dimethylvaleronitrile), tert-butylazo-2-cyanobutane; dibenzoyl peroxide, tert-butyl per[oxy]-2-ethylhexanoate, tert-butyl per[oxy]maleate, 2,2-azobis(isobutyronitrile); bis(tert-butylperoxy)cyclohexane, tert-butylperoxyisopropyl carbonate, tert-butyl per[oxy]acetate; 2,2-bis(tert-butylperoxy)butane, dicumyl peroxide, di-tert-amyl peroxide, di-tert-butyl peroxide, pinane hydroperoxide, cumene hydroperoxide, and tert-butyl hydroperoxide.
These polymerization initiators are used in amounts of about 0.05 to 10% by weight of the monomers, and, preferably about 0.1 to 5% by weight of an initiator is required.
t-Butylperoxy pivalate (TBPP) is a widely used free radical polymerization initiator: see U.S. Pat. Nos. 3,624,123; 3,887,528 and 4,237,257. TBPP undergoes thermal homolysis to produce t-butoxy and t-butyl free radicals. ##STR1## whereupon .beta.-scission of the t-butoxy radical produces the methyl free radical: ##STR2##
Thus, the active free radical species for initiation of free radical polymerization are: ##STR3##
The methyl and t-butoxy free radicals, respectively, have high bond dissociation energies (BDE) of 104 and 105 kcal/mole. This means that these radicals can readily abstract a labile hydrogen atom from the polymer to convert an otherwise linear polymer into branched polymers which have lower water solubility and lower molecular weights than the linear polymers. In addition, the half-life of TBPP initiator, i.e. the time at a given temperature to effect a loss of one-half of the perester's active oxygen content, is a lengthy 24.6 hours at 50.degree. C. Accordingly, TBPP requires a high reaction temperature, e.g. 60.degree.-80.degree. C., to carry out the polymerization within a reasonable period of time.
Accordingly, it is an object of this invention to provide polymerization processes using a free radical initiator whose thermal homolysis reaction provides free radicals which undergo rapid .beta.-scission to produce active free radical species which are weak hydrogen abstractors, and, thus enable the formation of linear polymers having high degree of water solubility and high molecular weights.